It is sometimes desired to maintain an enclosed space at ultra high vacuum (UHV). Pressures below 10.sup.-10 torr can be achieved by cryosorption pumping of H.sub.2 and He in liquid helium cooled clean vacuum systems. Low pressures such as these are now commonly maintained by adsorbing the He and H.sub.2 with molecular sieves and activated carbon. However, molecular sieves and activated carbon have several drawbacks in this application including low pumping speed and the introduction of unwanted particulates to the enclosed system.
Molecular sieves and activated carbon have a physical structure in which pores are available for trapping He and H.sub.2. However, their pumping speed is limited because the pores are long and the gas molecules, once they enter the pore, block the whole pore area. The active area available for adsorption is therefore reduced and not much is available for further adsorption. The molecular sieve or activated carbon must then be taken to high temperatures to relieve the block, and this interrupts the ultra high vacuum.
Molecular sieves and activated carbon have the disadvantage of introducing unwanted particulates into the vacuum system. The particulates can make it impossible to develop a clean vacuum as is required in many applications. Activated carbon and molecular sieves are also difficult to dry completely and therefore contain a lot of water. This makes it very difficult to use molecular sieves and activated carbon to achieve very low pressures, such as 10.sup.-10 torr. If one could dry the molecular sieves or activated carbon completely then they would be effective for very low pressures, but drying them completely is a very difficult thing to do.